1. Field of the Invention
This invention relates generally to detection and control of overcurrent conditions. In particular, this invention relates to the detection and control of overcurrent conditions as sensed in the primary winding of an output transformer of a power supply which converts power from high to low voltage levels.
2. Description of the Prior Art
Most electronic devices operate from an AC power source of either 110 volts and 60 hertz, or 220 volts and 50 hertz. It is often the case that these electronic devices must in turn produce a constant low-level DC power. This is most often accomplished by an internal power supply which first converts the AC power to a high-level voltage which is subsequently converted to a lower level voltage.
The conversion from high-level DC voltage to low level DC voltage is often accomplished by applying the higher level DC voltage to the primary winding of a step-down transformer. The application of high-level DC voltage is moreover usually accomplished at a high frequency so as to cut down on the size and weight of the step-down transformer. Such a high frequency application to the relatively small transformer core can result in magnetic saturation unless the magnetic energy in the core is released by an opposite or cancelling flux. This is usually accomplished by closely regulating the high frequency application of the DC voltage so as to allow for the subsequent cancellation of the magnetic flux build-up prior to saturation.
The prior art has the problem that in the event of an overcurrent condition, the main power transformer goes into saturation with ensuing collector current spikes of the switching transistors. These spikes degrade the switching transistor life.